In order to obtain desired color balance in color printing, a number of variables must be considered. Color printing paper which normally contains three emulsion layers sensitive to blue, green and red light (or their complements) varies with the specific manufacturing technique employed in producing the same, and with age. Other color variables result from inproper film exposure, the developing process and the lens of the camera employed. The so-called white light used in printing may also vary in the composition of primary colors therein. The artist may also wish to create a particular impression such as a sense of warmth or coolness through the use of color balance. In addition, film manufacturers often employ an overall color or "mask", usually an orange cast, to take advantage of the optimum characteristics of the various dyes employed in the film and paper.
In the course of typical color printing a test print is first made to enable compensation for the aforementioned variables, and compensating filters are employed to eliminate the variables to the extent that a print having the desired color balance is obtained.
A method of readily determining the appropriate filter compensation to provide optimum color balance in the course of printing is disclosed in my U.S. Pat. No. 3,443,868.
While red, green and blue filters may be employed for color compensation, it is normally preferred to employ magenta, cyan and yellow filters since each of these absorbs a single primary color (green, red and blue respectively) and they are simpler to use. Thus, in controlling the ratio of green, red and blue light employed in the printing step, filters of suitable densities must be employed.
Among the various types of color filters are acetate filters or "color printing" or "cp" filters which are only placed between the light source and the negative since they are not optically clear. Still another type of filter are the gelatin filters, commonly referred to as "color correction" or "cc" filters which are placed either in the tray between the light source and the lens or beneath the lens. Also special lamp housings are available in some enlargers which provide a continuous adjustment in the transmitted light by the use of narrow band or dichroic filters or other "built-in" capabilities.
The correction filters must be kept scrupulously clean to work properly and are available in a wide range of graduated densities. The special lamp housings or color heads for enlargers permit continuous adjustment of the three colors from zero to 1.20 density. Such color heads, however, are generally quite expensive.
In accordance with this invention a unique method of sequential color addition and integration uses a variable color filter head comprising a cover disc divided into four 90.degree. segments or quadrantal sectors. The filter head is mounted for rotation beneath the enlarger lens. Two of the segments are opaque and two are cut out or transparent to form effectively apertured portions. Rotatable movable into the cut-out segments are opposed segments of high density filter material which are opposed sectors and can be called a "butterfly" configuration. A plurality of filters of different color may be stacked beneath the cover disc whereby each may rotate into the cut-out portions but also may be completely concealed beneath the cover segments. They are carried by a carrier comprising at least a periferal annulus and central hub of relatively rigid material. Assuming no filter material is disposed in the cut-out disc openings, one hundred percent of the raw white light is transmitted in an unaltered state with zero color correction when one of the open segments is aligned with the enlarger lens and no light is transmitted when an opaque segment is disposed under the lens. Thus when the head is rotated the integrated, light value is white and 50% of the unaltered total quanta over any given time interval reaches the underlying easel, if the interval is relatively long compared to the period of one filter head rotation.
Assuming it is desired to filter out blue light emanating from the enlarger lens to obtain desired color balance in the underlying paper being exposed, a yellow filter is rotated to fill a portion of the cut outs of the cover disc. A tab connected to the disc on which the opposed segments of the yellow filter are mounted is manually engaged to rotate desired portions of the yellow filter segments into the cover disc cut outs. An indicator mark on the tab will move relative to a scale on the cover disc annulus bordering the cut-out segments indicating the effective density of the filter material for the particular angular portions disposed in the cover disc cut outs or open quadrants.
The filter head may be readily secured in operative position on an enlarger lens by means of a universal mount construction or support hereinafter described in detail. The mount is yoke-like and has one leg supporting the filter assembly and the other secured to the lens mounting. In the course of printing, the filter disc assembly is initially rotated by hand on a low-friction needle bearing in the path or axis of the generally collinated light. The head will continue to spin on the bearing under the forces of inertia. The weight and weight distribution determine the total time of rotation but a time of three minutes or more is obtainable. Since color exposures can exceed 30 seconds and or even in duration, the spinning disc assembly produces an average integrated coloration in the light in accordance with the setting of the filter material. Because of the prolonged exposure, there is no degradation in the print resulting from the sequential exposure or the varying speed in the disc rotation. Even respinning during exposure causes no perceptible effect on the print.
It is an object of this invention to provide an improved method of color control in photographic exposure including the sequential addition and integration of a plurality of light quanta.
It is another object of this invention, to provide a color filter disc assembly which readily attaches to an enlarger lens holder for providing variable color correction and practicing the improved method.
It is another object of this invention to provide a variable color filter assembly which may be readily and inexpensively formed of readily available materials and which may readily be put in use with substantially all enlarger constructions presently in the field.
It is a further object of this invention to provide a color filter assembly which may be readily, manually adjusted to provide a desired degree of filter hue and density whereafter the assembly is easily, manually rotated in the course of the printing step.